Credit Facilities (Narrative) (Details) - USD ($) $ in Thousands		9 Months Ended
	Oct. 26, 2021	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Debt borrowing capacity		$ 600,000
Borrowings outstanding		350,000	$ 200,000
Stand-by letters of credit		50,900
Wells Fargo Bank, N.A. Mature May 14, 2024
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 300,000
Committed credit facilities maturity		May 14, 2024
Borrowings outstanding		$ 75,000
Interest rate swap facility, amount		$ 75,000
Line of credit facility, interest rate		0.76%
Interest rate swap facility, fixed interest		2.32%
Interest rate swap facility, maturity date		May 14, 2024
BMO Harris Bank N.A. Mature May 14, 2024
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 200,000
Committed credit facilities maturity		May 14, 2024
Borrowings outstanding		$ 75,000
Interest rate swap facility, amount		$ 75,000
Line of credit facility, interest rate		0.78%
Interest rate swap facility, fixed interest		2.36%
Interest rate swap facility, maturity date		May 14, 2024
BMO Harris Bank N.A. Term Loan Mature May 14, 2024
Line of Credit Facility [Line Items]
Fixed rate term loan maturity		May 14, 2024
Fixed rate term loan amount		$ 100,000
Fixed rate term loan interest rate		1.28%
Periodic payment, principal		$ 1,250
Additional Outstanding Debt Under Credit Facilities
Line of Credit Facility [Line Items]
Borrowings outstanding		$ 100,000
Line of credit facility, interest rate		0.76%
Subsequent Event | BMO Harris Bank N.A. Mature May 14, 2024
Line of Credit Facility [Line Items]
Proceeds from issuance of long-term debt	$ 50,000